main.dart

import 'dart:async';


import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';


void main() async {
  final core = Core();
  runApp(MaterialApp(
    home: App(core),
  ));
}

class App extends StatefulWidget {
  final Core core;

  const App(this.core);

  @override
  _AppState createState() => _AppState();
}

class _AppState extends State<App> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          widget.core.news = [News(0)];
        },
      ),
      body: StreamBuilder<List<News>>(
        initialData: widget.core.news,
        stream: widget.core.newsStream,
        builder: (ctx, value) {
          List<Widget> news = value.data
              .map(_newsToWidget)
              .toList();

          return ListView(
            children: news,
          );
        },
      ),
    );
  }

  Widget _newsToWidget(News n) {
    return NewsWidget(n, () => _onNewsChanged(n));
  }

  void _onNewsChanged(News n) {
    List<News> newList = widget.core.news
        .map((News curN) => n.id == curN.id ? News(n.id, isFavorite: !n.isFavorite) : curN)
        .toList();

    widget.core.news = newList;
  }
}

// ignore: must_be_immutable
class Core {
  var _news = <News>[News(0), News(1), News(2)];
  final c = StreamController<List<News>>.broadcast();

  set news(List<News> news) {
    _news = news;
    c.sink.add(_news);
  }

  List<News> get news => _news;

  Stream<List<News>> get newsStream => c.stream;

  void dispose() {
    c.close();
  }
}

// ignore: must_be_immutable
class News {
  final bool isFavorite;
  final int id;

  News(this.id, {bool isFavorite})
      : this.isFavorite = isFavorite ?? false;
}

class NewsWidget extends StatelessWidget {
  final News news;
  final VoidCallback valueChanged;

  NewsWidget(this.news, this.valueChanged);

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      child: Icon(news.isFavorite ? Icons.plus_one : Icons.minimize),
      onTap: _onTap,
    );
  }

  void _onTap() {
    valueChanged?.call();
  }
}